Generally, an air conditioning device for a vehicle includes a heat exchanger that is provided with a condenser exchanging refrigerant being at high temperature and pressure delivered from a compressor with an external air to thereby make the heat-exchanged refrigerant liquefied, and with an evaporator that enables the liquefied refrigerant to be varied into air being at a low temperature such that the air around the low temperature air becomes cool.
Each of the condenser and evaporator includes a plurality of tubes, each of which has a plurality of refrigerant passages through which the refrigerant is passed, a plurality of corrugated fins placed between the tubes in a form of wave, a pair of header tanks that connect the both ends of each of the tubes in such a manner as to communicate with the tubes, and inlet and outlet pipes disposed in each of the header tanks, to and from which the refrigerant flows.
At that time, the condenser of the heat exchanger as mentioned above is provided with the plurality of flat-shaped tubes, each of which has a multipassage formed therein. This is disclosed in Japanese Patent Publication No. 11-159985.
As shown in FIGS. 1 and 2, the above-mentioned conventional heat exchanger is provided with a plurality of heat exchanger tubes 11, each of which forms a plurality of refrigerant passages 15 or 21 therein, wherein the refrigerant passages 15 or 21 with a polygonal or circular section are connected with each other, disposed in the same direction.
The above-discussed conventional heat exchanger has had the following problems.
So as to improve the performance of the heat exchanger, typically, it is important to increase a heat transfer area where the refrigerant is heat-exchanged. To do this, there has been provided a method in which a hydraulic diameter is reduced.
Referring to the above-mentioned conventional heat exchanger as shown in FIGS. 1 and 2, the plurality of refrigerant passages 15 or 21 are disposed in the width direction of the heat exchanger tube 11, and if the ratio of the width w of each of the refrigerant passages 15 or 21 to the height h is set higher than 1 (that is, w/h>1), a wall thickness t becomes increase as the hydraulic diameter is set relatively low in the heat exchanger provided with the heat exchanger tube 11 having the same size.
As the wall thickness t increases, however, the weight of the heat exchanger tube 11 increases as well as the production cost is raised due to the unnecessary consumption of the material.
On the other hand, FIG. 3 shows another conventional heat exchanger, which is disclosed in Japanese Patent Publication No. 2000-111290.
As shown in FIG. 3, the above-mentioned conventional heat exchanger is provided with a multipassage type of flat tube 5 in which a plurality of generally oval refrigerant passages 2a that are spaced apart equally, inclined by a predetermined angle α against the direction of an axis y.
The conventional heat exchanger as mentioned above has failed to improve the heat transfer efficiency thereof.
If an extruding speed increases by a predetermined value more than during the extruding process of the tube manufacturing, in addition, the above-mentioned conventional type of the heat exchangers undesirably form a pin hole on the external side of each of the tubes such that the pin hole is not filled even in the brazing process thereof, which results in the increment of the generation of the defective heat exchanger.
To produce a good quality of heat exchanger, therefore, the tube should be manufactured only at the predetermined extruding speed, which of course will cause the productivity thereof to be undesirably low.